Electric arc carbon



March 27, 1934. E. LEMAIGRE ELECTRIC ARC CARBON Filed Nov. 19, 1952Fig.2

Fig.1

Patented Mar. 27, 1934 UNITED STATES OFFICE ELECTRIC ARC CARBON FranceApplication November 19, 1932; Serial-No. 643,485 In France December 4,:1931 6 Claims.

This invention relates to electric. arc carb n for lightingpurposes andmore particularly to the carbons used in cinematograph projectors andthe like.

Positive carbons of this kind generally consist of a shell ofpractically" pure carbon and a core which comprises in addition tocarbon, suitable mineralizing materials such as, for example, alkali oralkaline earth compoundsrare earth salts,

'10 etc. Generally, the carbon is tapered at one end so as to give itapproximately the shape whichit takes spontaneously in the course ofcombustion.

It has been found by experience that such carbons exhibit the followingdisadvantage. When ""2. new carbon is'compared witha carbon whichhasalready been burnt, it is found that the chemical composition of thecoreis not the same in the end region, because the combustion has alwaysdeprived the hot end of the core of th most ela..- tile mineralelements. The result is that the ignie. tion of a new carbon alwaysdepends, on thermal, chemical and electricalconditions which arevariable and different inthe course. of use, 1118 of the phenomena most.frequently observed during pearance in the core of one, or more holes,each .of which corresponds. to azone of low luminosity :and thereforegives rise to an irregular illumination of the cinematographic screenwhenthe car,- 730 bon is employed for picture projection.

The primary object of this inventionis to overcome thisdisadvanta e andthisis. achieved by slightly mineralizing the carbonaceous tip of thecarbon by impregnation with a suitable metallic salt. Practicalexperience has shown that such impregnation, when correctly effected,remedies the disadvantage.

In order that the invention may be better understood, reference will bemade to the following -description taken in conjunction with theaccompanying drawing in which Fig. l is a longitudinal sectional Viewillustrating diagrammatically a positive arc carbon as hithertoconstituted; and Fig. 2 is also a longitudinal sectional viewillustrating diagrammatically a similar carbon manufactured inaccordance with this invention.

In Fig. 1, the carbon consists of a shell A of practically pure carbonand a core B which comprises, in addition to carbon, suitablemineralizing materials such as alkali or alkaline earth compounds, rareearth salts, etc. C is the tapered end ordinarily given to the unusedcarbon.

In Fig. 2 the carbon illustrated is similar to that -in Fig. 1, exceptthat a zone D situated at the tip this initial disturbed period consistsinthe ap} (Cl. GI-13.4)

of the taper C'has been impregnated with metallic salts, in accordancewith this invention.

The impregnation ,of the zone D is effected by any well-known, means,such as, forinstance, by immersing the carbon tip in a suitableimpregnating bath either at ordinary temperature or at an elevatedtemperature. It maybe effected by placing thecarbon with its tipincontact witha porous substance impregnated with a solution ofthemineralizing metallic salt, this porous substance consisting, for.instance, of asbestos paper, blotting paper, felt, or the like absorbentmaterial. Theimpregnationmay also be effectedby dropping a regulatedvolume of the impregnating, solution onthe plane end ofthetip C. Fortreat: ing a number of carbon tips, a battery of nozzles, mayconveniently be providedunder whichpassi. the carbons, tips upwards.Thesolutionpene: trates into the mass of. the carbonaceous shell. bycapillary action.

In order to,i llustrate themanner. of, carrying. the invention intoeffect, the, followingv .example. is given, but it, is notto be regardedas limiting. the invention'to this precise. method.

An electrodefsuch' as isrepresented inFig. 2 consists of a shelljAhavingaibas'e offlampblack agglomerated and hardened bybaking. The core. Bconsists of a mixture ofcarbonaceouspowder or graphite powder with analkali salt, for in: stance, potassiumsilicatef The salt for impreg- 33.

hating the zone D; ofthertip C consistsof, potas:

sium carbonate or ,borate, and. is made. up into a solution containing80 parts by weight of water and 20 parts by weight of the impregnatingsalt.

Some felt is soaked with this solution and the 1 electrode is arrangedupright on its tip with the latter in contact with the felt. Contact ismaintained until the impregnating solution has penetrated the tip 0 tothe desired depth D. If impregnation is efiected in the cold, it hasbeen found that periods of the order of 5 to 20 minutes are sufiicientfor this method of treatment for lampblack electrodes having an apparentdensity of about 1.45.

It is obvious that a similar method may be used, whatever thecomposition of the shell A (lampblack, retort carbon, petroleum coke,graphite, etc.) and also the mineralization of the core B, whether thelatter comprises only alkali salts or other mineral materials such as,for instance, the alkaline earth fluorides, the fluorides or oxides ofthe rare earths, etc. The form of the tip C may also vary from a simplesmall chamfer to a point considerably tapered. The ratio of the diameterof the core to that of the shell may also vary within wide limits suchas, for instance, from 0.15 to 0.8, according to the purpose for whichthe electrode is to be employed. Further, the impregnating salts may beother than potassium carbonate and potassium borate, although the latterhave been proved by experience to be the most suitable for use in thecase of positive carbons for cinematographic purposes.

The depth of the zone D may be varied in accordance with the diameterand shape of the car bon and also with the electrical conditions underwhich it is to operate, but in all cases the zone of shell which ismineralized must not exceed. that which is burnt away duringapproximately the first 2 to 5 minutes of operation.

I have observed what I believe to be the function and mechanism ofaction of such impregnation. It is known that the pure carbon arc isrelatively resistant, whilst most of the metallic salts vaporized in theare considerably increase its conductivity. The result is that onignition, the arc is mainly fixed by the core since this alone comprisesmineral elements. The current density to which this core is thussubjected may become very great and the vaporization of the mineralmaterial take place with violence, so that the least break in the core,were it only visible under the microscope at the moment of ignition,becomes an active centre of vaporization and a hole soon -fprms at thissame place. Once the excess of mineralizers is vaporized, a state ofequilibrium is established and the combustion of the carbonprogressively levels the edges of the hole, so that the latter hascompletely disappeared after several minutes ignition.

In cinemas, this disadvantage is so well-known that operators alwaystake more care to make the points of their carbons by preliminaryignition for several minutes. However, with carbons treated according tothe invention, the current density to which the core is subjected at themoment of ignition is limited because the mineral materials vaporizedare drawn from both the core and the shell, so that holes are no longerformed, the regularity of operation is much more quickly established,such that the period necessary to make points is reduced underadvantageous conditions for the user who thus obtains greater securityand, at the same time, economy in carbons.

With reference to the foregoing explanation, it is to be understood thatthe invention is not to be regarded as being in any way limited thereby,

' disadvantage remedied by the invention is much as obviously otherexplanations may be possible. It is sufiicient to state that as a resultof carefully conducted experiments with the invention, the improvementsabove-mentioned have been obtained.

The invention has been explained above with reference to positive arccarbons, but it must be clearly understood that in certain casesnegative carbons having a slightly mineralized core can be subjected tothe same process when perfect stability of operation is desired.Moreover, the invention can also be applied to the two-core carbons foroperating on alternating current, but its application in this field israther limited as the more rarely met.

I claim:

1. Electric arc carbon comprising a mineralized carbon core and a carbonshell surrounding said core, said shell being slightly mineralized atits operative end, while the remainder thereof consists of substantiallypure carbon.

2. Electric arc carbon comprising a mineralized carbon core and a carbonshell surrounding said core, a small quantity of an alkali metal saltbeing dispersed throughout the operative end of said shell, while theremainder thereof consists of substantially pure carbon.

3. Electric arc carbon comprising a mineralized carbon core and a carbonshell surrounding said core, a small quantity of an alkali metalcarbonate being dispersed throughout the operative end of said shell,while the remainder thereof consists of substantially pure carbon.

4. Electric arc carbon comprising a mineralized carbon core and a carbonshell surrounding said core, a small quantity of an alkali metal boratebeing dispersed throughout the operative end of said shell, while theremainder thereof consists of substantially pure carbon.

5. Positive arc carbon comprising a mineralized carbon core and a carbonshell surrounding said core, a small quantity of potassium carbonatebeing dispersed throughout the operative end of said shell, While theremainder thereof consists 120 of substantially pure carbon.

6. Positive arc carbon comprising a mineralized carbon core and a carbonshell surrounding said core, a small quantity of potassium borate beingdispersed throughout the operative end of said

